Integrated circuits include capacitors for a wide variety of uses, including filtering, storing information, power conditioning, tuning, etc. Memory devices such as Dynamic Random Access Memory (DRAM) and embedded-DRAM store bits of information in an array of memory cells formed from storage capacitors. Each storage capacitor is capable of storing one or more bits of information. A DRAM memory cell typically includes an access transistor coupled to a capacitor which can be buried in a semiconductor substrate (e.g., in a trench) or formed on the substrate surface. Information is written to the storage capacitor by activating the word line coupled to the gate of the access transistor. The storage capacitor is then either positively or negatively charged via the bit line of the cell to store the information. The information can be subsequently read out of the memory array by activating the word line and sensing the voltage level of the bit line.
Capacitor-based memory devices typically employ SiO2 or other materials having a similar dielectric constant as the capacitor node dielectric. SiO2 can be readily formed by oxidizing silicon, yielding a uniform and conformal oxide layer having high interface quality. However, the dielectric constant of SiO2 is relatively low (approximately 3.9), thus limiting the storage capacity of the device. Low capacitor storage capacity becomes problematic for low-voltage, high-performance semiconductor technologies. Mainly, it is difficult to accurately sense a small amount of charge stored on a capacitor. High-k dielectric materials such as hafnium and zirconium silicates and oxides (e.g., HfSiON, HfO2, HfSiO, HfSiON, etc.) and other materials or stacks of materials (e.g. ZrO2/SiO2/ZrO2, ZrO2/Al2O3/ZrO2, etc.) having a relatively high dielectric permittivity can be used in place of SiO2 as the capacitor dielectric film. Capacitors formed from high-k dielectric materials have a much higher charge storage capacity than their SiO2 counterparts and thus can be sensed more reliably and can store the same amount of charge in a smaller area.
However, storing the same charge on a high-k dielectric capacitor for long periods of time subjects the capacitor to undesirable transient effects. These transient effects degrade the capability of the capacitor to subsequently store charge of the opposite state. That is, the capacitor weakly stores charge of a new state when charge of the opposite state was previously stored by the capacitor for a relatively long period of time.
For example, high-k dielectric materials are more susceptible to charge trapping and/or polarization effects than lower-k dielectric materials such as SiO2. Electric dipoles tend to align in high-k material when an external electric field is applied to the capacitor electrodes. High-k materials are also subject to electronic trapping states in the band gap of the high-k material. When an electric field is applied for a sufficient time electrons tunnel from the capacitor electrodes into trapping states, creating charged states in the high-k dielectric material. The number and magnitude of electric dipoles that become aligned in the high-k material and the amount of electrons that tunnel into trapping states in the high-k dielectric material increase the longer a cell holds the same charge. In addition, the quality of the high-k dielectric material degrades more rapidly over time in the presence of aligned electric dipoles and trapping states.